Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2001-11-09
2003-10-14
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S027000
Reexamination Certificate
active
06633419
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the technology of holograms, and more particularly to a method and apparatus for efficiently producing a hologram having excellent embedability with a microscopic surface area and thickness thereof, high diffraction efficiency and therefore enhanced applicability to various recording substrates. The present invention also relates to a product incorporating such a hologram.
BACKGROUND ART
High energy density no fewer than 1 TW (10
12
W)cm
3
may be obtained by a femtosecond laser. When a light having such a high energy density is irradiated onto a material, high-density electrons will be excited in a short time period in the irradiated material. The energy of the excited electrons is converted into the vibrational energy of ions in the material within one nanosecond. Once the vibration energy density exceeds a given threshold, the ions break away from the material, resulting in an abrasion of the material. The abrasion caused in the material generates microscopic holes, and thereby the effective refractive index of the material is locally varied. This phenomenon is referred to as “micro-abrasion”. In this connection, when the vibration energy density is slightly lower than the threshold at which a destruction or abrasion is caused in the material, the material will not go far enough to be destroyed but will cause a variation in the refractive index of the material in connection with a variation or structural change in the atomic arrangement of the material.
It has been known to irradiate a high peak energy femtosecond laser beam to be converged at a spot having a small area onto a transparent crystal material, such as silica glass, BK7 optical glass, plastic (acrylic), quartz crystal, or sapphire, to cause an abrasion in the material so as to create fine holes, or to form micropores within the material through a nonlinear refractive index effect, or to vary the refractive index of the material through the structural change of atomic arrangement in the material.
For example, E. N. Glezer and E. Hazur: Appl. Phys. Lett. 71,882, (1997), and K. Miura, J. Qie, H. Inoue, T. Mitsuya and K. Hirano: Appi. Phys. Lett. 71,3329, (1997) reports that an optical waveguide may be formed by increasing the refractive index at an arbitrary location in an amorphous material such as silica glass. Japanese Patent Laid-Open Publication No. Hei 11-267861 discloses a method for forming a marking in a glass material. It has also been known to produce a diffraction grating by forming a number of spots in a regular arrangement using a device for irradiating a femtosecond laser beam onto a transparent material.
However, the application of this production method of diffraction gratings to actual elements and apparatuses involves unacceptable insufficiency. Further, limited few materials may vary the refractive index therewithin. In particular, as to a diamond crystal, any variation of the refractive index has not been achieved by this method.
The practical application of a titanium-sapphire laser has opened a way to obtain a femtosecond laser beam having a high coherence. Heretofore, it has been reported that when a femtosecond laser beam was irradiated onto a thin-film material formed of diamond or the like, a ripple pattern and/or a so-called Newton ring phenomenon caused likely by pulse interference were recorded in the material (A. M. Ozkan et al; Appl. Phys. Lett. 75,3716, (1999)), and this has suggested the coherence of the femtosecond laser beams. However, the reason for generating such a microstructure has not been clarified. Further, it has not been positively attempt to take advantage of the coherence of the titanium-sapphire laser.
A hologram has been conventionally produced through a two-beam exposure optical system by use of a gas laser output a high coherent continuous beam, and a recording substrate formed of a photosensitive organic substance or inorganic compound. However, the low energy density of such a gas laser has led to unmercifully long recording time and has forced to limitedly use a recording substrate having high photosensitivity. While a pulse laser, such as a ruby laser, has been used to cut down the recording time, it is indispensably required to combinationally use the photosensitive material as the recording matrix or substrate. In addition, it has been difficult to produce an embedded type hologram or a microhologram having a surface area of about 100 &mgr;m diameter or less.
DISCLOSURE OF INVENTION
Means for Solving the Problem
Heretofore, no report on a development of the two-beam exposure apparatus has been made, partially because it has been not clear if the coherence of the conventional femtosecond laser beam could be maintained sufficiently to enable the hologram recording. For example, a pulse beam having a pulse width of 100 femtoseconds is a short duration equivalent to a distance of only 30 &mgr;m, and its converged spot size is necessarily arranged in about 100 &mgr;m diameter in order to provide a high energy density. Further, the coherence of the high-density pulse can be degraded due to a nonlinear optical effect of a recording substrate during the propagation of the pulse through the substrate.
In view of the above conditions, the present invention provides a newly developed two-beam hologram exposure process in stead of the conventional laser beam irradiation process using the photosensitive material, to achieve a method capable of recording a hologram on a recording substrate essentially having no photosensitivity which is formed of a transparent organic or inorganic material, semiconductor material or metallic material, by use of a pair of pulse beams branched from a single pulse beam.
More specifically, according to the present invention, there is provided a method for producing a hologram using a two-beam laser interference exposure process comprising the steps of using as a light source a femtosecond laser having a pulse width of 900-10 femtoseconds and a peak output of 1 GW or more and capable of generating a pulse beam at or close to the Fourier transform limit, dividing the pulse beam from the laser into two by a beam splitter, controlling the two beams temporally through an optical delay circuit and spatially using both a mirror having a planar reflection surface (hereinafter referred to as “plane mirror”) rotatable slightly or finely and a mirror having a concave reflection surface (hereinafter referred to as “concave mirror”) rotatable slightly or finely to converge the beams on a surface of or within a substrate for recording a hologram at an energy density of 100 GW/cm
2
or more with keeping each polarization plane of the two beams in parallel so as to match the converged spot of the two beams temporally and spatially, whereby a hologram is recorded irreversibly on the substrate formed of a transparent material, semiconductor material or metallic material based on a variation in the configuration of the substrate and/or a variation in the refractive index of the substrate in connection with an abrasion of the substrate or a structural change in the atomic arrangement of the substrate caused by the high density energy irradiation.
Preferably, the light source includes a femtosecond laser having a pulse width of 500-50 femtoseconds and a peak output of 10 GW or more and, more preferably, capable of generating a pulse beam close to the Fourier transform limit. Preferably, the controlled beams are converged at an energy density of 1 TW/cm
2
or more. For example, given that the refractive index of the substrate is 1.5, a pulse width of 100 femtoseconds corresponds to a spatial distance of 20 &mgr;m and thereby provides a hologram having a total thickness of 10 &mgr;m or less. The position or range of the depth of the hologram may be controlled by changing at least one of optical path lengths of the two beams through the optical delay circuit, and the total thickness of the hologram may be adjusted by changing the pulse time of each beam.
A titanium-sapphire laser beam pulse may be generated substanti
Hirano Masahiro
Hosono Hideo
Kawamura Kenichi
Sarukura Nobuhiko
Amari Alessandro V.
Armstrong Westerman & Hattori, LLP
Japan Science and Technology Corporation
Robinson Mark A.
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